1. Field of the Invention
The present invention relates to a tungsten Chemical Vapor Deposition method.
2. Description of the Related Art
In the tungsten Chemical Vapor Deposition (CVD) method, the reduction of WF.sub.6 using hydrogen, silicon and silane is generally used. The H.sub.2 reduction of WF.sub.6 permits forming a tungsten film of a satisfactory quality with a sufficiently high growth rate. However, a large amount of by-product, hydrogen fluoride, is produced in the reaction. As a result, a silicon oxide film tends to be etched, making the H.sub.2 reduction of WF.sub.6 unsuitable for the fabrication of transistor. In addition, the surface-bonding strength between the tungsten film and the silicon substrate tends to be insufficient depending on the CVD conditions.
The Si reduction of WF.sub.6 permits improving the surface-bonding strength between the tungsten film and the substrate, compared with the H.sub.2 reduction of WF.sub.6. However, the growth rate of the tungsten film is low in the case of the Si reduction of WF.sub.6. Also, the maximum thickness of the tungsten film formed is determined by the self-limiting nature of the reaction. It follows that it is difficult to obtain a tungsten film of a desired thickness. What should also be noted is that, in the Si reduction of WF.sub.6 a tungsten film is grown while consuming Si. In other words, the surface region of the silicon substrate tends to be encroached. It follows that it is not desirable to employ the Si reduction of WF.sub.6 in the case of a shallow junction or a thin device region. Under the circumstances, any of the H.sub.2 reduction of WF.sub.6 and the Si reduction of WF.sub.6 is not practically employed in general.
The SiH.sub.4 reduction of WF.sub.6 is widely employed nowadays. However, the SiH.sub.4 reduction of WF.sub.6 is also defective in that it is difficult to obtain a sufficiently enough surface-bonding strength between the tungsten film formed and the substrate depending on the surface conditions. To overcome the difficulty, a pretreatment is applied under the same reaction chamber, using, for example, SF.sub.6 so as to slightly etch the surface region of the silicon substrate, before the deposition.
In other case, it is possible to form W silicide at the interface region between a silicon substrate and a tungsten film. However, it is difficult to find good conditions of silicide formation. In addition, the reproducibility in, for example, the thickness of the film is unsatisfactory.